First off, I'm glad I'm not "the village idiot" since this has happened to others.
It was in my haste, as I was taking the filter on and off (between shooting in bright sunlight and then in tents). Although I'm really disappointed in the threading of either the lens or the filter at this point (I'm leaning towards blaming the latter).
Secondly, just so everyone knows, I've tried a flat-head screwdriver into the knurling, but just with my own hand pressure (not tapping it with something like a hammer, which is too much vibration IMHO -- someone suggested that on another board, yikes!). That didn't work.
I was visualizing one of those rubber oil filter tools. That has now been confirmed as a good approach by several now (including another board).
Peter -- your suggestion to use a rubber jar opener is a great idea as well, and can't hurt to try! I think I might try that first.
BTW, I don't think I cross-threaded it, it clearly seems to have a "gap" on one side, and "flush" on the other -- showing that it just went on wrong, but possibly didn't ruin the threading.